Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on the intensity of the echo return. In a so-called "color flow" mode, the flow of blood or movement of tissue can be imaged. Conventional ultrasound flow imaging methods use either the Doppler principle or a time-domain cross-correlation method to estimate the average flow velocity, which is then displayed in color overlaid on a B-mode image.
Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The frequency shift of backscattered ultrasound waves may be used to measure the velocity of the back-scatterers from tissue or blood. The change or shift in backscattered frequency increases when blood flows toward the transducer and decreases when blood flows away from the transducer. The Doppler shift may be processed to estimate the average flow velocity, which is displayed using different colors to represent speed and direction of flow. The color flow velocity mode displays hundreds of adjacent sample volumes simultaneously, all color-coded to represent each sample volume's velocity.
In accordance with a known imaging system, the color flow mode employs multiple transmit firings for each focal point. Operating on a packet of as many as 16 transmits, a high-pass wall filter rejects echoes from slow-moving tissue or vessel walls to reduce the signal dynamic range for subsequent flow processing, using the Kasai autocorrelation algorithm or a cross-correlation algorithm to estimate the average flow velocity.
Although quantitative velocity information may be obtained in conventional color-flow imaging, the ability to see physical flow is limited by its clutter rejection capability, resolution, frame rate, and axial-only flow sensitivity.
Digital subtraction methods have been previously proposed to image moving reflectors in B-mode imaging (see Ishihara et al., "Path Lines in Blood Flow Using High-Speed Digital Subtraction Echography," Proc. 1992 IEEE Ultrason. Symp., pp. 1277-1280, and Ishihara et al., "High-Speed Digital Subtraction Echography: Principle and Preliminary Application to Arteriosclerosis, Arrhythmia and Blood Flow Visualization," Proc. 1990 IEEE Ultrason. Symp., pp. 1473-1476). However, these methods use frame-to-frame subtraction, which is essentially a two-tap wall filter with an extremely low cut-off frequency. The low cutoff frequency is due to the long time delay between adjacent frames, which does not adequately suppress signals from slow-moving tissue or vessel walls.
U.S. Pat. No. 5,632,277 to Chapman et al. discloses a nonlinear imaging system using phase inversion subtraction. The Chapman patent uses "first and second ultrasound pulses that are alternatively transmitted into the specimen being imaged," and mentions the particular embodiment of transmitting and summing on receive two pulses that differ by 180 degrees.
Conventional ultrasound images are formed from a combination of fundamental and harmonic signal components, the latter of which are generated in a nonlinear medium such as tissue or a blood stream containing contrast agents. In certain instances ultrasound images may be improved by suppressing the fundamental and emphasizing the harmonic signal components.
Contrast agents have been developed for medical ultrasound to aid in diagnosis of traditionally difficult-to-image vascular anatomy. For example, the use of contrast agents is discussed by de Jong et al. in "Principles and Recent Developments in Ultrasound Contrast Agents," Ultrasonics, Vol. 29, pp. 324-380 (1991). The agents, which are typically microbubbles whose diameter is in the range of 1-10 micrometers, are injected into the blood stream. Since the backscatter signal of the microbubbles is much larger than that of blood cells, the microbubbles are used as markers to allow imaging of blood flow. One method to further isolate echoes from these agents is to use the (sub)-harmonic components of the contrast echo, which are much larger than the harmonic components of the surrounding tissue without contrast agent. [See, e.g., Newhouse et al., "Second Harmonic Doppler Ultrasound Blood Perfusion Measurement," Proc. 1992 IEEE Ultrason. Symp., pp. 1175-1177; and Burns, et al., "Harmonic Power Mode Doppler Using Microbubble Contrast Agents: An Improved Method for Small Vessel Flow Imaging," Proc. 1994 IEEE Ultrason. Symp., pp. 1547-1550.] Flow imaging of (sub)harmonic signals has largely been performed by transmitting a narrowband signal at frequency f.sub.0 and receiving at a band centered at frequency 2f.sub.0 (second harmonic) or at frequency f.sub.0 /2 (subharmonic) followed by conventional color flow processing. This approach has all the limitations of a conventional color flow system, namely, low resolution, low frame rate and flow sensitivity only in the axial direction.
Thus, there is a need for a method of visualizing physical flow in B mode by directly imaging moving reflectors. This requires the imaging system to have high dynamic range, the ability to reject clutter from stationary or slow moving tissue and vessel walls, high resolution, high frame rate, and flow sensitivity in all directions.